Von Willebrand factor (VWF) is a multimeric glycoprotein that plays an essential role in the formation of platelet-rich thrombi, particularly under conditions of high shear stress. The main source of circulating VWF are the endothelial cells, where it is synthesized as a single chain pro-subunit with a discrete domain architecture, which was recently re-annotated: D1-D2-D′-D3-A1-A2-A3-D4-C1-C2-C3-C4-C5-C6-CK (1). Intra-cellular processing results in the removal of the propeptide (D1-D2 domains), and multimerization of the protein via amino- and carboxy-terminal disulfide bonding (2). These processes generate a heterologous pool of differentially sized multimers that may contain as many as 60 subunits, which are stored in the endothelial Weibel-Palade bodies and secreted into plasma via constitutive and agonist-induced pathways (3).
The multimer size of VWF is pertinent to its platelet-recruiting function, with the larger multimers displaying the highest hemostatic potential. In normal plasma, the multimers may contain between 2 and 40 subunits (1-20 bands when analyzed by SDS-agarose electrophoresis; (4)), which are shorter than those found in the endothelial storage organelles. Indeed, mechanisms are in place regulating VWF multimer size in the circulation, including proteolysis by the VWF-cleaving protease ADAMTS13 (A Disintegrin And Metalloprotease with ThromboSpondin domains-13) (5).
The molecular basis of VWF proteolysis by ADAMTS13 has been extensively studied (6). A number of interactive sites have been identified, including the A2- and D4-domains of VWF, which are important for the proper alignment of the ADAMTS13 active site (7-9). This active site attacks the Tyr1605-Met1606 peptide bond that is located within the VWF A2 domain (10). Interestingly, access to this peptide bond relies on the shear stress-induced unfolding of the substrate (for review see (11)), thereby protecting it against degradation while circulating normally, and limiting proteolysis to specific conditions. One of these conditions occurs when multiple VWF multimers assemble into large stretched bundles attached to the endothelial surface upon agonist-induced secretion. These bundles are not only capable of catching platelets, but their stretched conformation allows proteolysis by ADAMTS13 thereby preventing the release of platelet-binding ultra-large VWF multimers into the circulation (12, 13). Another condition favoring proteolysis occurs when VWF binds to platelets within the growing thrombus. This opens the ADAMTS13 cleavage site, and subsequent proteolysis results in reduction of VWF multimer size (14). Consequently, the platelet-binding capacity is diminished, avoiding excessive thrombus growth that could occlude the vessel.
The physiological relevance of ADAMTS13-mediated proteolysis of VWF is apparent from the severe thrombotic micro-angiopathy that is associated with the functional deficiency of ADAMTS13, a disorder known as thrombotic thrombocytopenic purpura (TTP) (15). In contrast, increased proteolysis of VWF by ADAMTS13 leads to a bleeding diathesis, as is exemplified by von Willebrand disease (VWD)-type 2A. VWD-type 2A is caused by mutations (mostly in the VWF A2 domain) that result in excessive proteolysis of the VWF by ADAMTS13, with a concomitant loss of the higher hemostatically active multimers (16). Another example relates to acquired von Willebrand syndrome (aVWS), where increased VWF proteolysis has been associated with various clinical settings, like aortic stenosis (17, 18) and ventricular assist devices (VADs) therapy (19, 20). Both conditions are characterized by an increased incidence of bleedings, especially gastrointestinal (GI) bleedings (20, 21), in association with a selective loss of high molecular weight (HMW)-multimers and an increase in VWF degradation products (18, 22). aVWS usually resolves after aortic valve replacement or discontinuation of VAD support (18, 20, 23, 24). However, the latter is limited by graft availability and GI bleeding represents thus the most challenging complication under VAD support.
Overall there is a clear unmet medical need for a treatment targeting specifically excessive degradation of HMW-VWF multimers, for instance induced by circulatory assist devices. Ideally, such treatment should only partially interfere with VWF degradation in order to prevent drug-induced TTP-like symptoms.